Large civil structures are frequently exposed to severe dynamic loading from several sources including earthquakes and high winds. During high winds, the sway motion at the top of a tall building and the vertical deflection on long suspension bridges may reach tens of feet. Therefore, one of the most important problems facing civil engineers today is to find ways to reduce the motions of a large civil structure to ensure structural integrity and human comfort.
Until recently, large civil structures have been built as passive structures. The external dynamic loads were resisted solely by the mass and stiffness of the structure. However, as the structures have become longer, taller and more flexible, and the demand for safety has increased, the need for building structures with some degree of adaptability to external forces has been recognized.
In the last two decades, structural control concepts have received considerable attention for the design of large civil structures. Several tall buildings have been constructed with various types of movement control devices installed. Most of these movement control devices are passive devices. The most commonly used passive systems are base isolation, viscoelastic dampers, and tuned mass dampers.
Viscoelastic dampers are installed in the World Trade Center buildings in New York, in the Columbia Center building, and a new building at 2 Union Square; in Seattle. Tuned mass dampers are installed in the Centrepoint Tower in Sidney, Australia, the Canadian National Tower in Toronto, the John Hancock Tower in Boston, and the Citicorp Center in New York. Tuned liquid dampers were recently installed in several buildings in Japan, including the Yokohama Marine Tower, the Shin Yokohama Prince Hotel, and a new control tower at the Narita Airport. Model test results indicate that tuned liquid dampers are effective in reducing wind-induced vibrations.
Liquid dampers have long been used to reduce the roll motion of ships. A typical antirolling tank will have an H configuration when viewed from above. The horizontal channel which connects the two wing tanks is designed to control the speed of the flow. Some of the antirolling tanks have also incorporated semi-active control devices to improve their effectiveness. The principles employed to achieve semi-active control is to adjust the flow through the horizontal channel by valves. Thus, the range of adjustment is very limited but is adequate for ship roll control..
Passive control devices are tuned to particular frequencies. A passive control device is thus only effective if the forcing frequency is close to the device's tuned frequency. Excitations that affect large civil structures are often multi-frequency forces. For example, seismic excitations have energies spread over a band of frequencies. When the excitation is a multi-frequency force, passive control devices are much less effective. Active control devices are needed to improve damping effectiveness against multi-frequency excitation forces. Several active structural control devices have been developed and installed. These active devices include active tendon systems and active mass dampers.
Active mass dampers are usually installed on the top floor of a tall building. They have a solid mass of several hundred tons (at least one percent of the building mass). The motion of this mass is regulated by hydraulic actuators during an earthquake so that the building motion can be reduced. The effectiveness of control devices can be improved by the addition of such active control. However, current active mass damper systems have many drawbacks. For example, these systems have an excessive peak power requirement. There are also reliability problems inherent with infrequently used equipment.
Tuned liquid dampers are similar to tuned mass dampers. Tuned liquid dampers utilize a large mass of liquid. As discussed above, tuned liquid dampers are only effective when the forcing frequency is near the natural frequency of the system. Tuned liquid dampers could be, made responsive to different forcing frequencies by utilizing active control. However, if the conventional active control concept were used, for example, to regulate the motion of the tank, peak power requirements and reliability problems would again be the severe limitations.
It is an object of the invention to provide active control of a tuned liquid damper system with a minimum power requirement.
It is another object of the invention to provide an actively tuned liquid damper system that is simple to construct and has relatively low cost in relation to previous structural motion control systems.
It is another object of the invention to provide an actively tuned liquid damper system that may be easily adjusted or altered during or after installation.
It is a further object of the invention to provide an actively tuned liquid damper system with multiple degrees of freedom.
It is a further object of the invention to provide an actively tuned liquid damper system that may be regularly tested without imparting motion to the structure to which the system is attached.